1. Field of the Invention
The present disclosure relates to a slip ring.
2. Description of the Related Art
A slip ring is widely used as a means for transmitting power or a signal from one part to another part via a rotating part such as a joint of a robot arm, for example. A slip ring includes a rotating shaft and brushes and typically has the following structure.
The rotating shaft is inserted into rings, which are made of a conductive material, and the rings are concentrically attached to and supported by the rotating shaft. Cables are connected to the rings. In addition, a brush fixing member for fixing the brushes, which are also made of a conductive material, is arranged at a certain distance from the rotating shaft. In this way, the brushes are fixed to contact the rings.
The rings and the brushes contact each other and are held so that the rings and brushes can slide on each other. In this way, power or a signal that is input from one of the ring side and the brush side is transmitted to the other side via contacts.
The contacts where the brushes and rings contact each other are always slid on each other. Consequently, since sliding surfaces are deteriorated by friction or the like, desired power and a desired signal may not be transmitted. To respond to such issue, Japanese Patent Application Laid-Open No. 9-28060 discusses a monitoring system capable of detecting deterioration of a sliding surface based on an increase in contact resistance, monitoring the state of a sliding contact of a slip ring, and notifying a user of the state. According to Japanese Patent Application Laid-Open No. 9-28060, a power supply circuit and a shunt resistor are connected in series with a slip ring, and a meter relay, a sequencer, and an alarming device are connected in series with the shunt resistor. With this configuration, change of a current value, which is caused when the contact resistance of the slip ring is increased, can be converted into a voltage by the shunt resistor, and the voltage can be detected.
However, the above slip ring has a problem that change in wiring resistance between the slip ring and the shunt resistor and change in contact resistance caused by deterioration of the sliding surface cannot be separately detected. Namely, the deterioration of the slip ring cannot be independently detected. For example, when the slip ring is installed at a joint of a machine such as an industrial robot arm, fatigue/deterioration of wirings may be caused by flexion of wirings as the robot arm operates. As a result, the wiring resistance may be changed. Thus, the slip ring monitoring system discussed in Japanese Patent Application Laid-Open No. 9-28060 cannot distinguish the fatigue/deterioration of wirings from the deterioration of the slip ring. In addition, for example, if the length of a wiring is increased and noise is mixed in the voltage from the power supply circuit, change in the contact resistance cannot be accurately detected.